In recent years, small mobile products equipped with an image pickup device (a camera module), such as a mobile phone, have become widely popular, and taking pictures instantly with such small mobile products has become a common practice. For image pickup devices that are incorporated in such small mobile products, an image pickup lens composed of four lenses is proposed (see Patent document 1, for example). This image pickup lens is small and can ensure a wide angle of view. In addition, various aberrations associated with the lens can be corrected favorably. Thus, the image pickup lens is compatible with a high-pixel image pickup element (i.e., can improve the image quality of captured images).
The image pickup lens described in Patent document 1 comprises, in order from the object side to the image surface side: a first lens that is a biconvex lens having positive power; a second lens that has negative power and whose lens surface facing the image surface side is concave; a third lens that is a meniscus lens having positive power and whose lens surface facing the image surface side is convex; and a fourth lens that has negative power, whose lens surfaces are both aspherical and whose lens surface facing the image surface side is concave near the optical axis. The image pickup lens satisfies the following conditional expression (5), where R22 is the radius of curvature of the lens surface of the second lens facing the image surface side and f is the focal length of the overall optical system.0.2<R22/f<0.6  (5)
Meanwhile, to further enhance the performance of image pickup devices used for small mobile products such as a mobile phone, there are demands for a small image pickup lens that can provide a wide angle of view of up to about 70° or higher in addition to being compatible with a small and high-pixel image pickup element.